MRAM has tremendous potential as a nonvolatile, solid-state memory to replace flash memory and electronically erasable programmable read-only memory (EEPROM). To enhance the performance of MRAM chips, it is necessary to reduce the lithographic minimum printable size. However, ion beam etching (IBE), a.k.a. ion milling, has been essentially the only method available for creating fine patterns, e.g., submicron patterns, in magnetic thin film structures. Because of the lack of volatile compounds for ferrous metals other than carbonyl, reactive-ion etching (RIE) has not been a viable technique for patterning thin magnetic films; and a RIE process based on carbonyl chemistry has not yet been developed. Thus, a chemical etching technique for patterning magnetic thin films based on Fe, Co and Ni is attractive because of the thin film nature of MRAM magnetic films (20–50 Å z-direction) relative to the x-y dimensions of patterned magnetic tunnel junction (MTJ) elements. The MRAM structure represents a complex multilayer system which includes numerous magnetic thin film layers. A typical MRAM structure is shown in FIG. 1. Specifically, the thin film structure shown in FIG. 1 comprises Si substrate 10, SiOx layer 12, a 150 Å Ti layer 14, Ni81Fe19 (40 Å) layer 16, Ir20Mn80 (120 Å) layer 18, Co90Fe10 (20 Å) layer 20, Al2O3 (10 Å) layer 22, Ni81Fe19 (40 Å) layer 24 and Ti (100 Å) layer 26. In this prior art magnetic structure, Al2O3 layer 22 serves as a tunnel barrier between the top magnetic film layer, i.e., Ni81Fe19 layer 24, and antiferromagnetic layer 18 and magnetic layers 16 and 20 which are present beneath the tunnel barrier layer. Layer 26 is a passivating layer that prevents moisture, air or other contaminants from entering into the structure, while layer 14 is an adhesion layer.
As is well known to those skilled in the art, the magnetic films of prior art MRAM structures, such as illustrated in FIG. 1, are quite thin. Patterning of the MRAM structure of FIG. 1 is typically carried out in the prior art by first applying a mask to the MRAM structure and patterning the mask by lithography (exposure and development). FIG. 2 shows the structure after these steps wherein reference number 28 represents the patterned mask. The pattern is then transferred to the MRAM structure by first removing the top 100 Å Ti film 26 of the MRAM structure by RIE, IBE, or wet etching. Next, the exposed Ni81Fe19 (40 Å) layer 24 can be pattern-wise etched by RIE, IBE, or wet etching. In a traditional wet etching process, a standard aqueous acid solution, such as sulfuric and/or nitric acid, is employed to etch the exposed Ni81Fe19 (40 Å) layer 24. Although the acid etchants are capable of etching through the exposed top magnetic layer 24 of the structure, the acid etchants are not selective for removing just that exposed magnetic layer 24. Instead, when the acid etchants are employed, they also etch the underlying alumina tunnel barrier layer 22, the Co90Fe10 layer 20, and some Mn in the Ir20Mn80 layer 18 providing the structure shown in FIG. 3.
Despite being capable of etching numerous magnetic layers in the MRAM structure, the use of prior art aqueous acid solutions causes Galvanic-coupling-accelerated dissolution of the Co90Fe10 (20 Å) 20 layer in the film region under the mask which is unacceptable for many applications. A desirable situation would be to etch through the top passivating layer 26 and the top exposed magnetic layer, i.e., layer 24, stopping at the thin Al2O3 layer 22, thereby leaving the underlying layers, i.e., layers 16, 18, and 20, unetched.
U.S. Pat. No. 6,426,012 describes a process of selectively etching the top magnetic layer of a magnetic structure stopping on the alumina tunnel barrier via the use of a dicarboxylic acid aqueous etchant solution. However, the acids employed in U.S. Pat. No. 6,426,012 are limited to weak acids which are subject to limited solubilities.
There is thus a need for developing another etching process which is capable of selectively etching the magnetic thin film structure so as to provide a patterned structure wherein the pattern is not formed in the tunnel barrier layer. Such an etch process would be beneficial since it would prevent unwanted Galvanic corrosion of the inner magnetic layers, while being able to pattern the top magnetic film layer and the tunnel barrier layer of the structure.